Generally, when a semiconductor laser is operated at a high power output, catastrophic optical damage (COD) occurs at an emitting facet of the laser and the laser is destroyed. This is caused by absorption by the laser facet part of the active layer of light generating heat thereby to decrease the energy band gap of that active layer part, which, in turn, again generates heat, resulting in repeating this operation. Therefore, many structures have been devised to prevent COD.
FIG. 6(a) is a perspective view, partially broken away, illustrating a prior art laser structure shown in "A Very Narrow-Beam AlGaAs Laser with a Thin Tapered-Thickness Active Layer (T.sup.3 Laser)", IEEE J. Quantum Electron., Vol. QE-23, pp. 712-719 (1987) by T. Murakami, K. Ohtaki, H. Matsubara, T. Yamawaki, H. Saito, K. Isshiki, Y. Kokubo, A. Shima, H. Kumabe, and W. Susaki. This laser has a structure in which an active layer thickness in a laser facet region is different from an active layer thickness in an inner region, and which is called a thin tapered-thickness (hereinafter referred to as T.sup.3) laser. In the Figure, on a surface of a p-type GaAs substrate 101 is formed a wide ridge 108 in the inner region of an element and narrow ridges 109 in the vicinity of the facets. An n-type GaAs current blocking layer 102 is disposed on the substrate 101 according to the configurations of the ridges produced on the substrate 101 so as to have also ridges 108 and 109 by itself. A striped V-groove 107 is formed extending in the resonator length direction in the center of the ridges 108 and 109 of the current blocking layer 102, and a bottom part of the V-groove 107 reaches the substrate 101. A p-type AlGaAs lower cladding layer 103 is disposed on the current blocking layer 102 and the V-groove 107, a p-type AlGaAs active layer 104 is disposed on the lower cladding layer 103, an n-type AlGaAs upper cladding layer 105 is disposed on the active layer 104 and an n-type GaAs contact layer 106 is disposed on the upper cladding layer 105. A p-side electrode 111 is disposed on the rear surface of the substrate 101 and an n-side electrode 112 is disposed on the surface of the contact layer 106. The feature of this prior art laser resides in that the laser includes the wide ridge 108 in the inner region, and the narrow ridges 109 in the vicinity of the laser facets and the active layer 104 on the ridges is thick on the wide ridge 108 and thin on the narrow ridges 109.
Next, a description is given of a production process of the T.sup.3 laser with reference to FIGS. 7(a) to 7(c).
First, as illustrated in FIG. 7(a), by processing the p-type GaAs substrate 101 by etching or the like, the wide ridge 108 is formed in a portion corresponding to the inner region of the element and the narrow ridges 109 are formed in portions corresponding to the regions in the vicinity of the facets. As illustrated in FIG. 7(b), crystal growth is carried out by MOCVD (Metalorganic Chemical Vapor Deposition), to produce the n-type GaAs current blocking layer 102 on the substrate 101 on which these ridges are formed. Secondly, as illustrated in FIG. 7(c), the striped V-groove 107 is formed extending in the resonator length direction in the center part of the current blocking layer 102 on the ridges 108 and 109 so that a bottom part of the V-groove 107 may reach to the substrate 101. Thereafter, crystal growth is carried out by LPE (liquid phase epitaxial growth), to produce successively the p-type Al.sub.0.48 Ga.sub.0.52 As lower cladding layer 103, the p-type Al.sub.0.15 Ga.sub.0.85 As active layer 104, the n-type Al.sub.0.48 Ga.sub.0.52 As upper cladding layer 105 and the n-type GaAs contact layer 106, on the current blocking layer 102 and the V-groove 107. Here, when the layers 103 to 106 are successively laminated by LPE on a wafer of such configuration as illustrated in FIG. 7(c), namely, the wafer having a wide ridge part and narrow ridge parts and having the striped V-groove in the center thereof, the active layer 104 on the wide ridge 108 is thick and the active layer 104 on the narrow ridges 109 is thin. In FIG. 6(a), a T.sup.3 structure 110 is shown as a structure in which the active layer has different thicknesses.
FIG. 6(b) is a side view of the T.sup.3 structure 110. When the active layer 104 is grown, for example, the thickness t2 on the wide ridge part may be 700 to 800.ANG., the active layer thickness t1 on the narrow ridge part is approximately 300 to 350.ANG.. In such a structure, since the active layer thickness on the laser facet is thin, confinement of laser beam in the active layer becomes weak and most of beam goes into the cladding layers 103 and 105. Thereby, optical density in the active layer at the laser facet is reduced and the COD level is increased. Here, the phenomenon that the active layer becomes thick on the wide ridge 108 and thin on the narrow ridges 109 is particular to the crystal growth performed by LPE, and cannot be realized by the MOVD method generally employed in GaAs wafer processing.
FIG. 8 is a perspective view showing another prior art laser structure described in ELECTRONICS LETTERS 26th April 1984 Vol.20 No.9, pp.383-384. This laser has window regions formed by disordering the vicinity of the facets of a multi-quantum-well (MQW) active layer. An n-type AlGaAs lower cladding layer 122 is disposed on an n-type GaAs substrate 121, a GaAs/AlGaAs MQW active layer 123 is disposed on the lower cladding layer 122, a p-type AlGaAs upper cladding layer 124 is disposed on the active layer 123 and a p-type GaAs cap layer 125 is disposed on the upper cladding layer 124. In the vicinity of both facets of the laser, Zn-diffused regions 128 reaching from the surface of the cap layer 125 to the lower cladding layer 122 are provided and the MQW in each Zn-diffused region 128 is a disordered region 129 which comprises AlGaAs of uniform composition due to mixed crystallization of GaAs and AlGaAs. The cap layer 125 and a portion of the upper cladding layer 124 are formed in a ridge shape. An n-side electrode 126 is disposed on the rear surface of the substrate 121 and a p-side electrode 127 is disposed on the surface of the cap layer 125. Reference numeral 150 designates a light emitting region and numeral 151 designates an emitted laser beam.
Next, a description is given of a production process of the window structure laser with reference to FIGS. 9(a) to 9(c).
First, as illustrated in FIG. 9(a), crystal growth is carried out by MBE (molecular beam epitaxy), to produce the n-type AlGaAs lower cladding layer 122, the GaAs/AlGaAs MQW active layer 123, the p-type AlGaAs upper cladding layer 124 and the p-type GaAs cap layer 125, on the n-type GaAs substrate 121. Secondly, a Si.sub.3 N.sub.4 film 130 is formed on a wafer and apertures are formed in the vicinity of regions which become the laser facets in a later cleavage process, by patterning the Si.sub.3 N.sub.4 film 130. As illustrated in FIG. 9(b), Zn 135 is diffused into the wafer through the apertures of the Si.sub.3 N.sub.4 film 130. As a diffusion method, either a vapor phase diffusion or a solid phase diffusion may be employed. Then, the diffusion is carried out at least until the diffusion front reaches the lower cladding layer 122 through the active layer 123. As a result, the MQW active layer 123 in the Zn-diffused region 128 becomes the disordered region 129 which comprises AlGaAs of uniform composition due to mixed crystallization of a GaAs well layer and an AlGaAs barrier layer. After then removing the Si.sub.3 N.sub.4 film 130, the p-side electrode 127 is formed on the cap layer 125 and the n-side electrode 126 is formed on the rear surface of the substrate 121. After etching the cap layer 125 and a portion of the upper cladding layer 124 to a desired shape, cleavage is conducted as illustrated in FIG. 9(c), to form the laser facets and to divide the wafer into chips, resulting in a completed laser as shown in FIG. 8.
In the semiconductor laser shown in FIG. 8, the vicinity of the facet in the active layer of the MQW structure is disordered by impurity diffusion and the disordered region 129 comprises AlGaAs having an energy band gap larger than that of a GaAs well layer. Therefore, a beam Generated in a portion where the MQW active layer is not disordered is not absorbed in the disordered region 129 and optical density in the vicinity of the facet is reduced, resulting in an increase in the COD level.
The semiconductor laser shown in FIGS. 6(a) and 6(b), has, as described above, controlled crystal layer thicknesses determined depending on the crystal growth mechanism inherent to the liquid phase growth, thereby reducing the optical density at the laser facet. However, there are problems in the liquid phase growth method, in that, a larger area substrate cannot be employed, it is inferior in mass production, and the crystal layer thickness is difficult to control.
In the semiconductor laser device shown in FIG. 8, in which the window structure is formed by disordering the vicinity of the facets of the MQW active layer, because a quantum-well active layer is employed for the active layer, the oscillation wavelength of the semiconductor laser is difficult to control. FIG. 10 is a diagram showing a relation between a well layer thickness (well width) and a photoluminescence wavelength equivalent to the oscillation wavelength of the active layer, in a single quantum well (SQW) active layer comprising a well layer of Al.sub.0.1 Ga.sub.0.9 As sandwiched by barrier layers of Al.sub.0.42 Ga.sub.0.58 As. As is apparent from the Figure, the oscillation wavelength is difficult to control because the wavelength shift relative to the well width is large at a well width less than 100.ANG..